This invention relates generally to thin film transistors and more particularly concerns a process in which high quality polysilicon films are produced at reduced temperatures compared to traditional methods of producing polysilicon films.
Polycrystalline silicon (polysilicon) thin-film transistors (TFTs) are important for a variety of large-area electronics applications due to their superior performance compared to amorphous silicon (.alpha.-Si) TFTs. Presently, the preferred method for obtaining high-quality polysilicon films with low surface roughness and high structural perfection on insulating substrates (e.g., oxide, quartz, glass) is by amorphous-phase deposition and subsequent thermal annealing. The crystallization process typically requires either annealing at temperatures ranging from 550.degree. C. to 600.degree. C. for periods of time greater than 2 hours or rapid (less than 1 second) thermal annealing at much higher temperatures of approximately 800.degree. C. This relatively large "thermal budget" required for crystallization is incompatible with large-area glass substrates, which cannot withstand prolonged exposure to temperatures much higher than 600.degree. C. due to warpage and shrinkage problems.
The crystallization of an .alpha.-Si film on an insulating substrate is limited by two processes. These two limiting processes are grain nucleation and grain growth. Nucleates must form before grain growth can take place. Nucleates will form primarily at the lower interface between the .alpha.-Si and the substrate. Once the nucleates have been formed, grain growth proceeds from the nucleating sites, both in the lateral direction (parallel to the interface) and the vertical direction (perpendicular to the interface). Since polysilicon films suitable for high performance TFT applications are typically less than 100 nm thick, with average grain sizes greater than 100 nm, the lateral grain growth rather than the vertical grain growth is the limiting step in the crystallization process.
A new method for obtaining high quality polysilicon films on insulating substrates is now proposed. In this method, crystallization occurs by epitaxial growth from an adjacent polycrystalline film. In this manner growth is limited only by the vertical (solid-phase) crystallization rate, rather than the grain-nucleation rate and the lateral grain-growth rate. Significant improvements in process throughput and reductions in annealing temperature are achievable for the crystallization process. These improvements mitigate the glass-substrate warpage and shrinkage problems with the older processes due to either long annealing times or high annealing temperatures.
Accordingly, it is the primary aim of the invention to provide a method of crystallization of polysilicon films which require lower temperatures and shorter times than traditional annealing processes.
Further advantages of the invention will become apparent as the following description proceeds.